This invention relates to improvements in techniques for depositing metal-sulphur coatings, for example MoS2, relates to metal-sulphur coatings with improved properties, and relates to articles coated with the new coating.
MoS2 coatings have been used as solid lubricants in a variety of applications, but mostly as coatings on bearings in aerospace applications. They are renowned for being soft and for deteriorating in water-containing atmospheres.
More recently, MoS2 coatings have been used to improve the efficiency of cutting tools. (German Patents 982616, or DE 2 345 355, and 4414051 C1. U.S. patent application Ser. No. 07 946 642, and Swiss Patent Application 2893/91).
MoS2 coatings can be deposited by various methods but the preferred method is sputtering. In the past MoS2 coatings deposited by sputtering have been poorly adhered, of low density, and with columnar structure.
MoS2 is conventionally very soft and powdery. It is most definitely not considered a hard coating. It can be scratched with a fingernail. Indeed, it is so soft that it is difficult to measure on the Vickers Hardness scalexe2x80x94it is right at the bottom end of the scale, and almost off the bottom end of the scale. It has a coefficient of friction that is very dependent upon humidity. In a dry atmosphere or in a vacuum it has a coefficient of friction of about 0.02. In humid atmospheres it has a higher coefficient of friction, and at atmospheric humidity it fails quickly because of the moisture in the air. This has restricted its use, for example to space-going applications where there is no water vapour present.
MoS2 is so soft that it cannot be used as a coating where the load is too high (above, say, 20N a pin on disc test with a 5 mm ball as the pin).
Recent developments at Teer Coatings using Closed Field Unbalanced Magnetron Sputter Ion Plating (CFUMBSIP) have produced MoS2 coatings of improved quality. In particular, the coatings are highly adherent, have a dense structure, a good wear rate, and are surprisingly hard. These coatings have the properties required for good tribological performance.
Our earlier patent application EP 0 521 045 discloses a closed field unbalanced Magnetron Sputter ion plating system suitable for performing the present invention, and its contents are hereby incorporated by reference. The reader is directed to read EP 0 521 045 A1 now to appreciate the disclosure incorporated. (xe2x80x9cClosed Fieldxe2x80x9d Magnetron Sputter Ion plating is the use of flux linking between adjacent magnetrons to provide a closed magnetic field between them, reducing the number of ionising electrons that escape from the system).
It has been found in practice that when using the sputtering method the results can be variable and that good coatings can be produced in one deposition sequence and better coatings in another deposition sequence which is intended to be identical.
We have linked some problems with variability in MoS2 coatings with the condition of the coating system and the condition of the MoS2 sputtering target, and in particular with the presence of contaminants in the deposition system. Two contaminants causing problems are water vapour from the atmosphere and Sulphur which is present in the chamber from previous MoS2 depositions. The source of water contamination is the humidity in the atmosphere. This can be absorbed on all surfaces within the coating chamber but in particular is readily absorbed by the MoS2 sputtering target.
Sulphur may also be present in the sputtering chamber following previous coating operations and can form compounds such as H2S which also contribute to contamination. The water vapour and/or the sulphur in the sputtering chamber can cause acidic contamination on the substrate surface prior to deposition leading to coatings with poor adhesion. Water vapour and/or sulphur compounds can also cause contamination during coating. The contaminants can lead to coatings with less desirable properties. For instance brittle coatings may be deposited due to the presence of contaminants.
The aim of the present invention is to produce a better molybdenum-sulphur coating.
According to a first aspect of the invention we provide a molybdenum-sulphur low friction coating having a thickness of at least 200 nm and a substantially pore-free homogenous non-columnar structure.
This has hitherto been entirely unachievable. Molybdenum-sulphur coatings have traditionally been thought of as very, very, soft. 500VHN is harder than stainless steel. We believe that our coating may be so much harder than has previously been achieved because it does not have pores, or voids.
One of the leading works on MoS2 films is by T Spalvins (see xe2x80x9cA Review of Recent Advances in Solid Film Lubricationxe2x80x9d by T Spalvins published in J Vac Sci Technol A5(2) March/April 1987). As described by Spalvins an MoS2 coating has an equiaxed structure overlapping a ridge-type structure. If the thickness of the equiaxed region is greater than about 200 nm it transforms above 200 nm into a columnar fibre structure with growth structure defined by voided boundaries, or pores. The equiaxed structure extending for perhaps up to 200 nm is pore free and is a densely packed structure, but above that fibres of a columnar structure grow, extending perpendicular to the substrate and are separated by open voided boundaries A few tens of nanometres wide. Such coatings are notoriously soft and break at the fibrous columnar region. FIG. 9 illustrates this hole-filled columnar structure.
The above is discussed in U.S. Pat. No. 5,268,216 of Keem et al who propose having multiple thin (less than 200 nm) layers of MoS2 to build up striated coatings with each layer being too thin for fibrous columnar growth to start.
We have found that by using our closed ring flux magnetron sputter ion plating system having a relatively high ion current density at the article to be coated we can create remarkable coatings, contrary to the teaching of Spalvins and Keem.
Preferably the coefficient of friction is 0.1 or less, and maybe 0.05 or less, and under some circumstances 0.005 or less.
MoS2 is also notoriously bad in even slightly humid atmospheres. Our MoS2 coating can operate successfully in normal air atmosphere. Our coating may have a coefficient of friction of about 0.02 when exposed to an atmosphere having 20% specific humidity, or 40% specific humidity.
In addition to producing pure, or substantially pure MoS2 coatings we have found that by incorporating another metal, or metal from a second target, we get even better coatings. The metal in the molybdenum sulphur material may be selected from the group;
Titanium; zirconium; hafnium
tungsten; niobium; platinum
vanadium; tantalum; chromium; gold; molybdenum.
We have been able to achieve coatings which incorporate Titanium with a Vickers hardness of at least 1000, and with a Vickers hardness of at least 1500, and with a Vickers hardness of at least 2000.
The ratio of molybdenum plus other metal to sulphur of maybe about 1:2. The other metal may be dissolved in the MoS2 crystal structure so as to have substantially no areas of elemental said other metal. The amount of said other metal may not be more than about 18% by weight (this is the figure for Titanium), but other metals may have different solubility in the MoS2 structure and have other upper limits.
The coating is preferably substantially homogeneous. It is preferably substantially amorphous. The coating may be such that substantially no crystal grains can be seen using a transmission electron microscope at atomic resolution.
The coating may have Titanium also present and have a composition MoxTiySz where x+y≈1 and Z≈2, and where X is at least 4 times Y, and where the coating is a homogeneous amorphous coating showing substantially no crystal grain boundaries when viewed with a transmission electron microscope at atomic resolution, and which has a coefficient of friction of no more than 0.1, and which shows substantially no discrete elemental Ti. The coating in most embodiments is non-columnar and has no pores.
There may be an adhesion layer, or, interloper, beneath a molybdenum-sulphur layer. The adhesion layer may comprise a layer of Titanium or tungsten.
The coating may have an adhesion to a steel article of at least 70N. The coating may have a wear rate of less than 10xe2x88x9216 M3/Nm, or of 10xe2x88x9217 M3/Nm, or better.
We can provide much thicker coatings of MoS2, or Mo/Ti/S than has previously been possible (with good hardness and wear rate). We can provide coatings with a thickness of 200-500 nm, or even up to 1000 nm or more.
According to a second aspect of the invention we provide molybdenum-sulphur-metal (Me) coating MoxTiySz wherein x+y is approximately equal to 1, Z is approximately equal to 2, Y is approximately 10% of Xxc2x19% of X, and wherein the coating is an unlayered coating with no crystal boundaries visible when viewed using a transmission electron microscope at atomic resolution, and wherein Me is a metal from the group: Titanium, zirconium, hafnium, tungsten, niobium, vanadium, tantalum, chromium and gold and wherein said coating has a Vickers hardness of at least 1,000, and which has a coefficient of friction of 0.03 or less in an air atmosphere of 10-20% humidity.
The coating may have a coefficient of friction of 0.1 or less in an air atmosphere with 20-40% humidity.
According to another aspect the invention comprises a Molybdenum Titanium Sulphur coating having no crystal boundaries visible when viewed at atomic resolution with a transmission electron microscope, and having an unlayered structure, and which when in an air atmosphere of 10-20 humidity has a Vickers hardness of at least 1,000 and a coefficient of friction of 0.03 or less, and a wear rate of 10xe2x88x9216 m/Nm or less.
The invention also comprises a dry machining tool, requiring no liquid lubricant to machine steel, the tool having a coating in accordance with any of the preceding aspects of the invention.
According to a further aspect the invention comprises a method of providing a molybdenum-sulphur coating on an article comprising operating a magnetron sputter ion plating system in an evacuation operation, followed by a backfill operation, followed by a cleaning operation, followed by a coating operation; said sputter system having a sputter chamber, a first target of MoS2-sputter material to be coated onto said article, and a second, cleaning target of metal; and in which in said evacuation operation the pressure in said sputter chamber is reduced to lower than 5xc3x9710xe2x88x924 torr; and in said backfill operation argon gas is allowed into said sputter chamber to a pressure of between 5xc3x9710xe2x88x924 torr and 10xe2x88x922 torr; and in said cleaning operation said second target is energised to produce a flux of reactive cleaning metal which reacts with impurities in said sputter chamber so as to remove them from having an active presence during the subsequent coating operation, and wherein during said cleaning operation a negative voltage is applied to said article to be coated such that, the size of the negative voltage applied to said article being sufficiently high, and the power applied to said second target being sufficiently low, ions from said second target bombard said article to be coated, but there is no net deposition of second target films on said article during said cleaning operation and in which during the coating operation the negative bias voltage on said article to be coated is reduced so that net deposition of material on the said article occurs, and wherein said molybdenum-sulphur coating deposited has an unlayered non-columnar structure.
Preferably, the coating has a hardness of at least 1,000VHN.
Preferably said metal target is energised during said coating operation to provide metal ions, and wherein the molybdenum-sulphur coating comprises metal held on the molybdenum-sulphur lattice to a concentration up to the solubility of said metal in the molybdenum sulphur lattice. Preferably said metal is Titanium and it is deposited in said coating so that the coating has up to 18% Titanium. During said cleaning operation a negative voltage of between 400V and 1,000V is preferably applied to said article to be coated. During deposition the negative voltage bias applied to the article is typically between 25V and 100V.
The invention also comprises a MoxTiySz coating having a thickness of at least 300 nm, and a Vickers Hardness of at least 1000, a wear rate of about 10xe2x88x9217 M3/Nm or better, a coefficient of friction of about 0.02 or better in an air atmosphere with 20% humidity wherein X≈1xe2x88x92Y, Z≈2, y is in the range of 0-0.18, and wherein said coating is substantially homogeneous throughout its depth, and wherein the coating is a dense substantially unvoided coating having substantially no columnar structure.
According to a further aspect the invention comprises a MoxTiySz coating having a thickness of at least 300 nm, a Vickers hardness of at least 1000, a wear rate of about 10xe2x88x9217 M3/Nm or better, a coefficient of friction of about 0.05 or better, wherein Me is a metal from the group: Titanium, zirconium, hafnium, tungsten, niobium, vanadium, tantalum, chromium, molybdenum and gold; and wherein x≈1xe2x88x92Y, Y is in the range 0 to 0.2, Z≈2, and wherein said coating is a dense substantially unvoided coating having substantially no columnar structure.
One way in which we achieve a good MoxTiySz coating (where x+y≈1 and z≈2 and Me=a metal such as Titanium, or Tungsten) is by the use of a second sputter target (or targets) as well as the metal/sulphur target (or targets) (e.g. metal sulphide such as MoS2 or WS2).
The target (or targets) can be of any metal but is preferably a highly reactive metal such as Titanium. In the following a Titanium target is referred to. The sputter electrodes can be simple diodes, RF electrodes or magnetrons, but magnetron electrodes are preferred.
During the ion cleaning of the substrates which normally precedes the deposition process in the ion plating technique, and during coating we introduce the concept of energising the second target of (e.g.) Titanium.
We energise the Titanium target for two reasons. Firstly in order to be able to strike an intense discharge to the substrates at a low argon pressure so that we ion clean the substrates efficiently at low argon pressure. Secondly we energise the Ti target to mop up the water vapour which is absorbed by the MoS2 target and also by the MoS2 previously deposited on the chamber walls, and the sulphur compounds (e.g. H2S) which remain in the chamber following previous depositions. This mopping up or gettering of the water vapour and sulphur compounds during the ion cleaning part of the process helps to create very good adhesion between coating and substrate. It also reduces contamination during deposition which can cause brittleness and other undesirable properties.
We also prefer to energise the Ti target during the coating operation which introduces some Titanium into the MoS2 coating. We obtain an improvement in the adhesion of the coating which may be a function of the ion cleaning.
Since making the present invention we have become aware of some prior art (most of which are the results of an EPO search). The results of the search support the inventiveness of the present invention.
JP 3 014 227 is perhaps the most relevant document. This discloses a method of making semiconductor devices (a different field from low-friction coatings) in which a Titanium target is energised in an argon atmosphere cleaning stage of operation. The Argon atmosphere is cleaned in the sputtering chamber prior to sputtering on to an aluminium substrate. The document is concerned solely with purifying the argon gas, not the aluminium target (aluminium is not a source of contamination). In our invention the target to provide the coating (MoS2) is a cause of the water vapour problem and a source of sulphur contamination. JP 3014227 does not have an ion cleaning stage of the system to bombard the substrate to be coated with ions prior to coating. It is not even an ion plating system (just sputtering, no ion plating acceleration of ions onto the substrate).
JP 3 014 227 does not envisage cleaning the substrate to be coated and the target of coating material with ion cleaning (prior to ion plating the substrate) in order to remove impurities associated with the target. Moreover, it does not suggest using its Titanium target to reduce the argon pressure necessary to ion clean (it does not ion clean). It does not energise the Titanium target during the coating operation.
DE-C-44 14 051 discloses a sputtering technique in which MoS2 is deposited on an interlayer of either Cr3Si or Cr. It does not discuss ion cleaning or ion plating. It does not have a second target of metal (e.g. Titanium).
DD-A-20 28 98 discusses depositing MoS2 on cutting tools, but does not discuss using a second metal target in impurity gettering stage, prior to ion cleaning a substrate before ion plating it, nor having a Ti target energised during coating and including Ti in the MoS2 coating.
DE-A-3 516 933 recognises that the presence of water vapour in the argon atmosphere is deleterious and describes a way of removing it from the gas atmosphere, but is not concerned with removing impurities from a plating chamber.
JP-A-3 097 855 shows a way of purifying argon gas prior to introducing to its sputter chamber. If used on MoS2 systems the purified argon gas would only be re-contaminated when it was introduced into the sputter chamber (because the MoS2 in the chamber has water vapour impurities, and other sulphur impurities would exist in the chamber). JP-A-3 097 855 shows that although a problem had been appreciated they had not thought of the present invention. Its effect is similar to DE-A-3 516 933 in this respect.
EP-A-0 534 905 is equivalent to CH 2893/91 referred to earlier, and merely discusses the desirability of MoS2 coatings. It has little relevance to the present invention which is concerned with being able to provide good coatings with repeatability and fewer inconsistencies. It does not discuss having Titanium in the coating.
U.S. Pat. No. 5,002,798 is concerned with depositing. MoS2 coatings, but was a totally different technique: it is not magnetron sputter ion plating, and does not include Titanium in the coatings.
FR-A-2 586 430 discusses the desirability of low friction coatings on article such as skis, and using vapour phase deposition techniques, but does not discuss magnetron sputter ion plating, or ways of reducing impurities in MoS2 coatings.
U.S. Pat. No. 5,268,216 is specifically in the field of MoS2 coatings and teaches the perceived impossibility of achieving thick non-fibrous/columnar MoS2 and teaches building up a thick coating of MoS2 using repeated thin layers to avoid producing a pored structure. We now are able to do what Keem and Spalvins (referenced earlier) thought was not possible.
According to one aspect of the invention we provide a sputter system having a first target (or targets) of metal sulphide (e.g. MoS2 or WS2) and a second target (or targets) of metal (e.g. Ti or W).
Preferably the first target is a metal disulphide. Preferably the metal second target is made of Titanium.
The first and second targets are each separately energisable (e.g. each have their own associated magnetron, which have independently controllable power supplies).
Preferably the arrangement is such that the system is capable of being activated in a cleaning operation in which the second target is energised. The first (e.g. MoS2) target can be energised but is preferably not energised during the ion cleaning operation.
Preferably the first target comprises a magnetron with a target element, as may the second target. The, one, or both, magnetrons may be unbalanced.
In a MSIP system the cleaning operation involves the bombardment of the substrates by ions (usually argon ions) and in the new arrangement also involves the bombardment of the second target with ions so as to produce metal neutrals and ions in the sputter chamber, the metal neutrals and ions react with any impurities in the atmosphere of the chamber so as to remove them from the atmosphere. The metal neutrals and ions may adhere to the walls of the chamber. For example, water and sulphur in the coating chamber atmosphere react with the metal, preferably Titanium, created from the second target, to form stable and non-contaminating compounds, during the cleaning operation. The ion bombardment, and the metal atmosphere created in the cleaning operation, help to react deposits of impurities left over from earlier coating operations. This improves the efficiency of the ion cleaning of the substrate and improves the quality of coating that is laid down.
At least some of the reactive cleaning metal may be ionised metal, but the metal is not necessarily ionised. It could simply be sputtered metal. In one embodiment the reactive cleaning metal is partially ionised (perhaps about 5% ionised). Preferably the second target is Titanium or alternative reactive metal. The first and/or second targets may have a respective associated magnetron.
The method may include having some of the metal of the second target incorporated into the layer that is deposited on the substrate to be coated.
The method may include introducing to the sputter chamber in the coating operation a further material which is incorporated in the coating. The further material is preferably gaseous at its point of introduction and may be nitrogen, or oxygen, or a hydrocarbon gas.
Water and/or sulphur in the sputter chamber atmosphere reacts with the metal, preferably Titanium, created from the second target to form stable and non-contaminating compounds during the cleaning operation.
According to an aspect of the invention we provide an article coated using the system of the first aspect of the invention, or using the method of a second aspect of the invention.
According to another aspect the invention comprises the use of second target of metal in a cleaning operation of a sputter system so as to reduce impurities in a low-friction coating subsequently produced from the system.
According to another aspect the invention comprises the use of a second metal target in a cleaning operation of a sputter system for the production of a low friction metal-sulphur, e.g. MoS2 coating with improved mechanical properties.
Preferably the coating is between 0.1 xcexcm and 10 xcexcm thick, preferably about 1 xcexcm thick.
According to another aspect of the invention we provide a method of reducing impurities when using a magnetron sputter ion plating system to coat an article with a metal sulphide coating having a target of metal sulphide to produce the coating and a second, different, metal target, and energising the second target in a cleaning operation before coating commences, the second target a) producing reactive metal neutrals and ions which getter impurities in the ion plating chamber and also b) facilitating the generation of an intense discharge at low inert gas pressure (in the ion plating chamber) to achieve better ion cleaning of the substrate to be coated than would be achieved if there were no energised second target.
We may provide a method of reducing contamination of a coating laid down by a magnetron ion plating system on a substrate using a first target of material to be deposited that attracts impurities (e.g. water vapour) comprising ion cleaning the substrate and the first target in an ion cleaning operation, prior to a deposition/coating operation, and wherein in the ion cleaning operation a second target of good gettering material is energised to create gettering neutrals or ions of said material which ions or atoms clean the system.
Our coating system may comprise a first sputter source and second sputter source, the first and second sputter sources being arranged close to each other so that they can sputter simultaneously on to substrates situated in front of the two sputter sources, the first and second sources being of different materials so as to produce coating fluxes of different materials that deposit on to the substrates simultaneously to produce a coating of an approximately homogeneous mixture of the two materials on the substrates.
FIG. 4 schematically illustrates such a system.
The substrates can be on a moving holder so that they are brought into and moved out of the coating position. The movement is preferably rotary.
It has been found that the coatings produced by the above described methods have a coefficient of friction that is consistently below 0.1, and frequently as low as 0.02.
The method is preferably applied to MSIP, and most preferably CFUBMSIP.
Preferably the movement is rotation of the substrate about an axis. Preferably the axis of rotation does not pass through the substrate, Alternatively it may pass through the substrate.